Best bet would be to look on Craig's List in your area for someone selling a fridge that runs well, for about $100. Once you have your fridge all you need to do is plumb it up with faucets, taps and co2. If you already have a co2 setup, you've already have one of the most expensive parts covered. Total you shouldn't pay more than $300 for the complete setup.

If space is an issue, and you can't use a full sized fridge, I used a 4.4 cu. ft. all fridge unit that I got at hhgregg for about $190. It can fit under counter, and with removing the shelves on the door and replacing it with plexiglass, you can fit two corny kegs and a CO2 canister in it. That is what I did when I built my bar and have been very happy with the set up.